A Pilot Study To Assess the Suitability of Riboflavin As a Surrogate Marker of Breast Cancer Resistance Protein in Healthy Participants

We recently showed that riboflavin is a selected substrate of breast cancer resistance protein (BCRP) over P-glycoprotein (P-gp) and demonstrated its prediction performance in preclinical drug-drug interaction (DDI) studies. The aim of this study was to investigate the suitability of riboflavin to assess BCRP inhibition in humans. First, we assessed the substrate potential of riboflavin toward other major drug transporters using established transfected cell systems. Riboflavin is a substrate for organic anion transporter (OAT)1, OAT3, and multidrug and toxin extrusion protein (MATE)2-K, with uptake ratios ranging from 2.69 to 11.6, but riboflavin is not a substrate of organic anion-transporting polypeptide (OATP)1B1, OATP1B3, organic cation transporter (OCT)2, and MATE1. The effects of BMS-986371, a potent in vitro inhibitor of BCRP (IC 50 0.40 μM), on the pharmacokinetics of riboflavin, isobutyryl carnitine, and arginine were then examined in healthy male adults (N = 14 or 16) after oral administration of methotrexate (MTX) (7.5 mg) and enteric-coated (EC) sulfasalazine (SSZ) (1000 mg) alone or in combination with BMS-986371 (150 mg). Oral administration of BMS-986371 increased the area under the plasma concentration-time curves (AUCs) of rosuvastatin and immediate-release (IR) SSZ to 1.38- and 1.51-fold, respectively, and significantly increased AUC(0-4h), AUC(0-24h), and C max of riboflavin by 1.25-, 1.14-, and 1.11-fold (P-values of 0.003, 0.009, and 0.025, respectively) compared with the MTX/SSZ EC alone group. In contrast, BMS-986371 did not significantly influence the AUC(0-24h) and C max values of isobutyryl carnitine and arginine (0.96- to 1.07-fold, respectively; P > 0.05). Overall, these data indicate that plasma riboflavin is a promising biomarker of BCRP that may offer a possibility to assess drug candidate as a BCRP modulator in early drug development. SIGNIFICANCE STATEMENT: Endogenous compounds that serve as biomarkers for clinical inhibition of breast cancer resistance protein (BCRP) are not currently available. This study provides the initial evidence that riboflavin is a promising BCRP biomarker in humans. For the first time, the value of leveraging the substrate of BCRP with acceptable prediction performance in clinical studies is shown. Additional clinical investigations with known BCRP inhibitors are needed to fully validate and showcase the utility of this biomarker.

The Role of Uptake and Efflux Transporters in the Disposition of Glucuronide and Sulfate Conjugates

Glucuronidation and sulfation are the most typical phase II metabolic reactions of drugs. The resulting glucuronide and sulfate conjugates are generally considered inactive and safe. They may, however, be the most prominent drug-related material in the circulation and excreta of humans. The glucuronide and sulfate metabolites of drugs typically have limited cell membrane permeability and subsequently, their distribution and excretion from the human body requires transport proteins. Uptake transporters, such as organic anion transporters (OATs and OATPs), mediate the uptake of conjugates into the liver and kidney, while efflux transporters, such as multidrug resistance proteins (MRPs) and breast cancer resistance protein (BCRP), mediate expulsion of conjugates into bile, urine and the intestinal lumen. Understanding the active transport of conjugated drug metabolites is important for predicting the fate of a drug in the body and its safety and efficacy. The aim of this review is to compile the understanding of transporter-mediated disposition of phase II conjugates. We review the literature on hepatic, intestinal and renal uptake transporters participating in the transport of glucuronide and sulfate metabolites of drugs, other xenobiotics and endobiotics. In addition, we provide an update on the involvement of efflux transporters in the disposition of glucuronide and sulfate metabolites. Finally, we discuss the interplay between uptake and efflux transport in the intestine, liver and kidneys as well as the role of transporters in glucuronide and sulfate conjugate toxicity, drug interactions, pharmacogenetics and species differences.

Sex-specific expression mechanism of hepatic estrogen inactivating enzyme and transporters in diabetic women

Circulating estrogens levels significantly decrease in menopause and levels off in postmenopausal women. Accordingly, the liver represses levels of enzymes and membrane transporters, thereby decreasing capability of inactivating and excreting estrogens. Women increasingly develop type 2 diabetes during or after menopause. Estrogens are known to promote liver diseases in these women. Here, we have found that the estrogen inactivating sulfotransferase (SULT1E1) and an ATP-binding cassette subfamily G member 2 (ABCG2), a gene encoding breast cancer resistance protein that exports sulfated estrogens, increased their expression levels in diabetic women but not men. For the sulfotransferase gene, phosphorylated nuclear receptors ERα and RORα, at Ser212 and Ser100, respectively, bind their response elements to activate the SULT1E1 promoter in women. This coordinated increase in estrogen inactivation and excretion, and the phosphorylated nuclear receptor-mediated gene activation could be a defense mechanism against toxicities of estrogens through inactivation and excretion in the livers of women.

OATP2B1 - The underrated member of the organic anion transporting polypeptide family of drug transporters?

The organic anion transporting polypeptide 2B1 (OATP2B1) was one of the first cloned members of the SLCO family. However, its physiological and pharmacological role is still poorly understood, and object of a current debate on the transporter's relevance. Within this commentary, we summarize the data currently available on the transporter's expression and its substrates and highlight the strength and difficulties of the methods that have been applied to gather these data. The conclusion drawn from these findings was that OATP2B1 due to its intestinal expression is most likely involved in oral drug absorption of its substrate and therefore prone for interactions. This has been tested in in vivo drug interaction and/or pharmacogenetic studies. While some of these support the notion of OATP2B1 being of relevance in drug absorption, the pharmacogenetic findings are rather inconclusive. We will explain our thoughts why OATP2B1 may not influence the general systemic pharmacokinetic of certain substrates, but possibly local distribution processes, like the transfer across the blood-brain-barrier. Besides the pharmacokinetic aspects, there are data on endogenous molecules like coproporphyrins and sulfated steroids. Therefore, we will also highlight possible physiological roles of OATP2B1, which are driven by its expression pattern in the tubular cells of the kidney as well as its expression in the blood brain barrier. Finally we also deal with the advantages and disadvantages in the use of animal models to decipher the role of OATP2B1 in pharmacokinetics of its substrates and beyond.

Ethinyl estradiol sulfate acts without fluid resuscitation through estrogen receptors to rapidly protect the cardiovascular system from severe hemorrhage

BACKGROUND

Our in vivo rodent and pig model evidenced that estrogen and its derivative, ethinyl estradiol sulfate (EES), promote survival following hemorrhagic shock. To determine its mechanism, we first confirmed EES binding to estrogen receptor (ER) and improving/restoring cellular signaling, countering the assumption that EES, an ethinyl estradiol metabolite, is inactive. In addition, we examined if EES acts rapidly, consistent with nongenomic signaling. We selected the biomarkers of cardiovascular performance, reduction of apoptosis and proinflammatory responses, and elaboration of nitric oxide (NO) to validate efficacy.

METHODS

A rat trauma-hemorrhage model, consisting of a midline laparotomy and controlled bleeding (60% blood loss) without fluid resuscitation, was used. At 30 minutes after hemorrhage, heart performance was monitored, and Western blots were used to quantify biochemical analytes. The specificity of EES for ER was profiled with ER antagonists. Binding studies by Sekisui XenoTech (Kansas City, KS) determined an LD50 value for EES binding the rat ER.

RESULTS

The EES IC50 value was 1.52 × 10-8 Mol/L, consistent with pharmacologic efficacy. Ethinyl estradiol sulfate raised mean arterial pressure and ±derivative of pressure over time (dP/dT) significantly (but did not fully restore) within a 30-minute window. Levels of apoptosis and activation of NF-κB were dramatically reduced, as was elaboration of nitric oxide (NO) by inducible nitric oxide synthase. Phospho-endothelial nitric oxide synthase (eNOS) was restored to physiological levels. The restoration of cellular signaling occurs before restoration of cardiac contractility.

CONCLUSION

Ethinyl estradiol sulfate is a potent drug for improving heart performance, which also dramatically reduces damage by apoptosis, proinflammatory activity, and NO production, validating that EES can blunt multiple harmful outcomes arising from hypoxia and hypovolemia. The actions are dependent on receptor engagement, where specificity is confirmed by ER antagonists. The constraint of a 30-minute sampling window affirms that the responses are nongenomic and very likely restricted to cell-surface receptor engagement. The rapidity of these responses makes EES promising for intervention in the "golden hour."

Characterization of the cellular transport mechanisms for the anti-cachexia candidate compound TCMCB07

BACKGROUND

Cachexia is a debilitating, life-threatening condition whose pathology includes reduced food intake accompanied by hypermetabolism, leading to a catabolic state. The hypothalamic melanocortin system is a critical regulator of metabolic rate with effects being mediated through the melanocortin-4 receptor (MC4R). MC4R activation is also critical to the initiation and maintenance of cachexia. A major problem in the design of anti-cachexia drugs has been the need to cross the blood-brain barrier to access the metabolic rate-controlling centres in the hypothalamus. The overwhelming majority of anti-cachexia drugs are only effective when administered intracerebroventricularly. TCMCB07 is a cyclic nonapeptide peptide MC4R antagonist with parenteral anti-cachexia activity in both small and large animal models. This suggests it can cross the blood-brain barrier. The aim of this study was to examine potential transport mechanisms of TCMCB07 furthering its preclinical development for subsequent studies in humans.

METHODS

In vitro studies were performed in transporter-transfected cells to study whether or not TCMCB07 was an inhibitor as well as substrate for OATP1A2, OATP1B1, OATP1B3, OATP2B1, OCT2, OAT1, OAT3, MATE1, MATE2-K, P-gp (MDR1), and BCRP. In vivo mass balance studies were also performed in mice to evaluate the absorption and disposition of TCMCB07 after oral and intravenous bolus administrations.

RESULTS

TCMCB07 inhibited the uptake of prototypical substrates in cells transfected with OATP1A2 (IC50 24.0 μM), OATP1B1 (IC50 6.8 μM), OATP1B3 (IC50 307 μM), OATP2B1 (IC50 524 μM), OCT2 (IC50 1,169 μM), MATE1 (IC50 8.7 μM), and MATE2-K (IC50 20.7 μM) but not in cells transfected with OAT1 and OAT3. TCMCB07 did not affect the P-gp (MDR1)-mediated and BCRP-mediated permeability of prototypical substrates in transfected cells. Importantly, direct evidence was shown for the uptake of TCMCB07 in OATP1A2-transfected cells (i.e. Vmax 236 pmol/mg, Km 58.4 μM, and Kd 0.39 μL/mg), demonstrating that the nonapeptide was a substrate for this transporter. Mass balance studies demonstrated that 24.2% of TCMCB07 was absorbed orally in vivo (P = 0.0033) and excreted primarily in the bile after both oral and intravenous administrations.

CONCLUSIONS

OATP1A2 is the transporter responsible for the oral absorption of TCMCB07 in the intestine and for its pharmacologic response in the brain.

Enhanced and Persistent Inhibition of Organic Cation Transporter 1 Activity by Preincubation of Cyclosporine A

Recent pharmacogenetic evidence indicates that hepatic organic cation transporter (OCT) 1 can serve as the locus of drug-drug interactions (DDIs) with significant pharmacokinetic and pharmacodynamic consequences. We examined the impact of preincubation on the extent of OCT1 inhibition in transfected human embryonic kidney 293 (HEK293) cells. Following 30-minute preincubation with an inhibitor, approximately 50-fold higher inhibition potency was observed for cyclosporine A (CsA) against OCT1-mediated uptake of metformin compared with coincubation, with IC₅₀ values of 0.43 ± 0.12 and 21.6 ± 4.5 µM, respectively. By comparison, only small shifts (≤2-fold) in preincubation IC₅₀ versus coincubation were observed for quinidine, pyrimethamine, ritonavir, and trimethoprim. The shift in CsA OCT1 IC₅₀ was substrate dependent since it ranged from >1.2- to 50.2-fold using different experimental substrates. The inhibition potential of CsA toward OCT1 was confirmed by fenoterol hepatocyte uptake experiment. Furthermore, no shift in CsA IC₅₀ was observed with HEK293 cells transfected with OCT2 and organic anion transporter (OAT) 1 and OAT3. Short exposure (30 minutes) to 10 µM CsA produced long-lasting inhibition (at least 120 minutes) of the OCT1-mediated uptake of metformin in OCT1-HEK293 cells, which was likely attributable to the retention of CsA in the cells, as shown by the fact that inhibitory cellular concentrations of CsA were maintained long after the removal of the compound from the incubation buffer. The potent and persistent inhibitory effect after exposure to CsA warrants careful consideration in the design and interpretation of clinical OCT1 DDI studies. SIGNIFICANCE STATEMENT: Preincubation of OATP1B1 and OATP1B3 with their inhibitor may result in the enhancement of the inhibitory potency in a cell-based assay. However, limited data are available on potentiation of OCT1 inhibition by preincubation, which is a clinically relevant drug transporter. For the first time, we observed a 50-fold increase in CsA inhibitory potency against OCT1-mediated transport of metformin following a preincubation step. The CsA preincubation effect on OCT1 inhibition is substrate dependent. Moreover, the inhibition potential of CsA toward OCT1 is confirmed by hepatocyte uptake experiment. This study delivers clear evidences about the potent and persistent inhibitory effect on OCT1 after exposure to CsA. Further studies are needed to assess the effect of CsA on OCT1 drug substrates in vivo.

Efflux transport of estrogen glucuronides by human MRP2, MRP3, MRP4 and BCRP

Estrone, estradiol and estriol are endogenous human estrogens that are rapidly conjugated with glucuronic acid in both intestinal and hepatic epithelial cells. The resulting glucuronides, estrone-3-glucuronide (E₁-G), estradiol-3- and 17-glucuronides (E₂-3G and E₂-17G), as well as estriol-3- and 16-glucuronides (E₃-3G and E₃-16G) are found in human plasma and urine. Unlike E₂-17G, the efflux transport of other estrogen glucuronides by human transporters has not yet been investigated comprehensively. We have studied the transport of E₁-G, E₂-3G, E₃-3G, E₃-16G and estrone-3-sulfate (E₁-S), another important estrogen conjugate, using the vesicular transport assay with recombinant human MRP2, MRP3, MRP4, MDR1 and BCRP that were expressed in insect cells. The transport screening assays revealed that whereas E₁-S was a good and specific substrate for BCRP, the less transporter-specific conjugates, E₁-G and E₂-3G, were still transported by BCRP at 10-fold higher rates than E₁-S. BCRP also transported E₃-16G at higher rates than the studied MRPs, while it transported E₃-3G at lower rates than MRP3. MRP2 exhibited lower or equal transport rates of E₁-G, E₂-3G, E₃-3G and E₃-16G in comparison to MRP3 and BCRP in the screening assays, mainly due to its high K_m values, between 180 and 790 μM. MRP3 transported all the tested glucuronides at rather similar rates, at K_m values below 20 μM, but lower V_max values than other transporters. In the case of E₃-3G, MRP3 was the most active transporter in the screening assay. MRP4 transported only E₃-16G at considerable rates, while none of the tested estrogen conjugates was transported by MDR1 at higher rates than control vesicles. These new results, in combination with previously reported in vivo human data, stimulate our understanding on the substrate specificity and role of efflux transporters in disposition of estrogen glucuronides in humans.

Fitting Transporter Activities to Cellular Drug Concentrations and Fluxes: Why the Bumblebee Can Fly

A recent paper in this journal argued that reported expression levels, k_cat and K_m for drug transporters could be used to estimate the likelihood that drug fluxes through Caco-2 cells could be accounted for solely by protein transporters. It was in fact concluded that if five such transporters contributed ‘randomly’ they could account for the flux of the most permeable drug tested (verapamil) 35% of the time. However, the values of permeability cited for verapamil were unusually high; this and other drugs have much lower permeabilities. Even for the claimed permeabilities, we found that a single ‘random’ transporter could account for the flux 42% of the time, and that two transporters can achieve 10 · 10⁻⁶ cm·s⁻¹ 90% of the time. Parameter optimisation methods show that even a single transporter can account for Caco-2 drug uptake of the most permeable drug. Overall, the proposal that ‘phospholipid bilayer diffusion (of drugs) is negligible’ is not disproved by the calculations of ‘likely’ transporter-based fluxes.

There has been recent debate as to the relative extents to which cellular transmembrane drug transports occur through any phospholipid bilayer region or is transporter-mediated only.

Much recent evidence suggests (perhaps surprisingly) that phospholipid bilayer diffusion is negligible.

A recent article in this journal suggested that the expression profile and kinetics of known transporters might not be adequate to explain the most active drug fluxes (of verapamil and propranolol) in Caco-2 cells via transporters only.

We show with our own simulations that this is not in fact the case, especially when evolutionary selection is taken into account, and that the Haldane relation accounts straightforwardly for directional differences, even for equilibrative transporters.

Typical protein transporters alone can easily account for measured drug fluxes in Caco-2 cells.

Transport of the placental estriol precursor 16α-hydroxy-dehydroepiandrosterone sulfate (16α-OH-DHEAS) by stably transfected OAT4-, SOAT-, and NTCP-HEK293 cells

16α-Hydroxy-dehydroepiandrosterone sulfate (16α-OH-DHEAS) mainly originates from the fetus and serves as precursor for placental estriol biosynthesis. For conversion of 16α-OH-DHEAS to estriol several intracellular enzymes are required. However, prior to enzymatic conversion, 16α-OH-DHEAS must enter the cells by carrier mediated transport. To identify these carriers, uptake of 16α-OH-DHEAS by the candidate carriers organic anion transporter OAT4, sodium-dependent organic anion transporter SOAT, Na(+)-taurocholate cotransporting polypeptide NTCP, and organic anion transporting polypeptide OATP2B1 was measured in stably transfected HEK293 cells by LC-MS-MS. Furthermore, the study aimed to localize SOAT in the human placenta. Stably transfected OAT4-HEK293 cells revealed a partly sodium-dependent transport for 16α-OH-DHEAS with an apparent Km of 23.1 ± 5.1 μM and Vmax of 485.0 ± 39.1 pmol/mg protein/min, while stably transfected SOAT- and NTCP-HEK293 cells showed uptake only under sodium conditions with Km of 319.0 ± 59.5 μM and Vmax of 1465.8 ± 118.8 pmol/mg protein/min for SOAT and Km of 51.4 ± 9.9 μM and Vmax of 1423.3 ± 109.6 pmol/mg protein/min for NTCP. In contrast, stably transfected OATP2B1-HEK293 cells did not transport 16α-OH-DHEAS at all. Immunohistochemical studies and in situ hybridization of formalin fixed and paraffin embedded sections of human late term placenta showed expression of SOAT in syncytiotrophoblasts, predominantly at the apical membrane as well as in the vessel endothelium. In conclusion, OAT4, SOAT, and NTCP were identified as carriers for the estriol precursor 16α-OH-DHEAS. At least SOAT and OAT4 seem to play a functional role for the placental estriol synthesis as both are expressed in the syncytiotrophoblast of human placenta.

Do MDR1 and SLCO1B1 polymorphisms influence the therapeutic response to atorvastatin? A study on a cohort of Egyptian patients with hypercholesterolemia

BACKGROUND

Statins are among the most prescribed drugs worldwide to reduce the risk of cardiovascular events. Interindividual variability in drug response is a major clinical problem and is of concern during drug development. Statins, such as atorvastatin, are taken orally and access to their site of action in the liver is greatly facilitated by both intestinal and hepatic transporters.

OBJECTIVE

To examine the impact of polymorphisms of the multidrug resistance 1(MDR1) and solute carrier organic anion transporter 1B1 (SLCO1B1) genes on the therapeutic response to atorvastatin as well as the presence of gender-gene interaction.

METHODS

Serum lipid levels were determined at baseline and 4 weeks following 40 mg/day atorvastatin treatment in 50 Egyptian hypercholesterolemic patients (27 males and 23 females). Identification of MDR1 C3435T and SLCO1B1 A388G gene polymorphisms was performed using a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method.

RESULTS

Treatment with atorvastatin resulted in a mean reduction of total cholesterol (TC), low density lipoprotein cholesterol (LDL-C), and triglyceride (TG) of 8.7 %, 9.2 %, and 4.1 %, respectively, and a mean increase of high density lipoprotein cholesterol (HDL-C) of 1 %. Baseline and post-treatment HDL-C levels were statistically significantly higher in the MDR 1 TT homozygotes when compared with the CC wild type. The percentage change in TC, LDL-C, TG, and HDL-C did not show any statistically significant difference when compared among the different MDR 1 C3435T or SLCO1B1 A388G genotypes. The SLCO1B1 GG homozygotes showed a decrease in TG, whereas there was an increase in TG following atorvastatin treatment in AA and AG carriers in females; however, males did not show any statistically significant difference. There was no statistically significant association between either the coronary artery disease (CAD) risk factors (family history of CAD, hypertension, diabetes mellitus, smoking) or concomitant medications with the percentage change in different lipid parameters.

CONCLUSION

MDR1 C3435T was associated with baseline and post-treatment HDL-C variation. SLCO1B1 A388G showed gender-related effects on TG change following atorvastatin treatment. None of the comorbidities or the concomitant medications influenced the percentage change of lipid parameters following atorvastatin treatment. The results of this study may lead to an improved understanding of the genetic determinants of lipid response to atorvastatin treatment.

Development of Novel, 384-Well High-Throughput Assay Panels for Human Drug Transporters

Transporter proteins are known to play a critical role in affecting the overall absorption, distribution, metabolism, and excretion characteristics of drug candidates. In addition to efflux transporters (P-gp, BCRP, MRP2, etc.) that limit absorption, there has been a renewed interest in influx transporters at the renal (OATs, OCTs) and hepatic (OATPs, BSEP, NTCP, etc.) organ level that can cause significant clinical drug-drug interactions (DDIs). Several of these transporters are also critical for hepatobiliary disposition of bilirubin and bile acid/salts, and their inhibition is directly implicated in hepatic toxicities. Regulatory agencies took action to address transporter-mediated DDI with the goal of ensuring drug safety in the clinic and on the market. To meet regulatory requirements, advanced bioassay technology and automation solutions were implemented for high-throughput transporter screening to provide structure-activity relationship within lead optimization. To enhance capacity, several functional assay formats were miniaturized to 384-well throughput including novel fluorescence-based uptake and efflux inhibition assays using high-content image analysis as well as cell-based radioactive uptake and vesicle-based efflux inhibition assays. This high-throughput capability enabled a paradigm shift from studying transporter-related issues in the development space to identifying and dialing out these concerns early on in discovery for enhanced mechanism-based efficacy while circumventing DDIs and transporter toxicities.

The SLC10 carrier family: transport functions and molecular structure

The SLC10 family represents seven genes containing 1-12 exons that encode proteins in humans with sequence lengths of 348-477 amino acids. Although termed solute carriers (SLCs), only three out of seven (i.e. SLC10A1, SLC10A2, and SLC10A6) show sodium-dependent uptake of organic substrates across the cell membrane. These include the uptake of bile salts, sulfated steroids, sulfated thyroidal hormones, and certain statin drugs by SLC10A1 (Na(+)-taurocholate cotransporting polypeptide (NTCP)), the uptake of bile salts by SLC10A2 (apical sodium-dependent bile acid transporter (ASBT)), and uptake of sulfated steroids and sulfated taurolithocholate by SLC10A6 (sodium-dependent organic anion transporter (SOAT)). The other members of the family are orphan carriers not all localized in the cell membrane. The name "bile acid transporter family" arose because the first two SLC10 members (NTCP and ASBT) are carriers for bile salts that establish their enterohepatic circulation. In recent years, information has been obtained on their 2D and 3D membrane topology, structure-transport relationships, and on the ligand and sodium-binding sites. For SLC10A2, the putative 3D morphology was deduced from the crystal structure of a bacterial SLC10A2 analog, ASBT(NM). This information was used in this chapter to calculate the putative 3D structure of NTCP. This review provides first an introduction to recent knowledge about bile acid synthesis and newly found bile acid hormonal functions, and then describes step-by-step each individual member of the family in terms of expression, localization, substrate pattern, as well as protein topology with emphasis on the three functional SLC10 carrier members.

Cynomolgus monkey as a potential model to assess drug interactions involving hepatic organic anion transporting polypeptides: in vitro, in vivo, and in vitro-to-in vivo extrapolation

Organic anion-transporting polypeptides (OATP) 1B1, 1B3, and 2B1 can serve as the loci of drug-drug interactions (DDIs). In the present work, the cynomolgus monkey was evaluated as a potential model for studying OATP-mediated DDIs. Three cynomolgus monkey OATPs (cOATPs), with a high degree of amino acid sequence identity (91.9, 93.5, and 96.6% for OATP1B1, 1B3, and 2B1, respectively) to their human counterparts, were cloned, expressed, and characterized. The cOATPs were stably transfected in human embryonic kidney cells and were functionally similar to the corresponding human OATPs (hOATPs), as evident from the similar uptake rate of typical substrates (estradiol-17β-d-glucuronide, cholecystokinin octapeptide, and estrone-3-sulfate). Moreover, six known hOATP inhibitors exhibited similar IC(50) values against cOATPs. To further evaluate the appropriateness of the cynomolgus monkey as a model, a known hOATP substrate [rosuvastatin (RSV)]-inhibitor [rifampicin (RIF)] pair was examined in vitro; the monkey-derived parameters (RSV K(m) and RIF IC(50)) were similar (within 3.5-fold) to those obtained with hOATPs and human primary hepatocytes. In vivo, the area under the plasma concentration-time curve of RSV (3 mg/kg, oral) given 1 hour after a single RIF dose (15 mg/kg, oral) was increased 2.9-fold in cynomolgus monkeys, consistent with the value (3.0-fold) reported in humans. A number of in vitro-in vivo extrapolation approaches, considering the fraction of the pathways affected and free versus total inhibitor concentrations, were also explored. It is concluded that the cynomolgus monkey has the potential to serve as a useful model for the assessment of OATP-mediated DDIs in a nonclinical setting.

Structural requirements of bile acid transporters: C-3 and C-7 modifications of steroidal hydroxyl groups

The apical sodium dependent bile acid transporter (ASBT) and sodium-taurocholate cotransporting polypeptide (NTCP) are potential prodrug targets, but the structural requirements for these transporters are incompletely defined. The objective of this study was to evaluate the effect of C-3 and C-7 substitution on bile acid interaction with these bile acid transporters. Nineteen bile acid analogs were tested against ASBT and NTCP for binding, as well as translocation. Results indicated that ASBT and NTCP accommodated a wide range of substituents for binding, but all major C-7 modifications resulted in analogs that did not demonstrate active uptake by either ASBT or NTCP. A C-3 modification that was not tolerated at C-7 still afforded translocation via ASBT and NTCP, confirming the relative unacceptability of C-7 modification. Both ASBT and NTCP demonstrated a generally similar binding potency. Results suggest that drug conjugation to the C-3 hydroxyl group, rather than C-7, has potential to lead to a successful prodrug targeting ASBT and NTCP.

Organic anion transporting polypeptide 1B1: a genetically polymorphic transporter of major importance for hepatic drug uptake

The importance of membrane transporters for drug pharmacokinetics has been increasingly recognized during the last decade. Organic anion transporting polypeptide 1B1 (OATP1B1) is a genetically polymorphic influx transporter expressed on the sinusoidal membrane of human hepatocytes, and it mediates the hepatic uptake of many endogenous compounds and xenobiotics. Recent studies have demonstrated that OATP1B1 plays a major, clinically important role in the hepatic uptake of many drugs. A common single-nucleotide variation (coding DNA c.521T>C, protein p.V174A, rs4149056) in the SLCO1B1 gene encoding OATP1B1 decreases the transporting activity of OATP1B1, resulting in markedly increased plasma concentrations of, for example, many statins, particularly of active simvastatin acid. The variant thereby enhances the risk of statin-induced myopathy and decreases the therapeutic indexes of statins. However, the effect of the SLCO1B1 c.521T>C variant is different on different statins. The same variant also markedly affects the pharmacokinetics of several other drugs. Furthermore, certain SLCO1B1 variants associated with an enhanced clearance of methotrexate increase the risk of gastrointestinal toxicity by methotrexate in the treatment of children with acute lymphoblastic leukemia. Certain drugs (e.g., cyclosporine) potently inhibit OATP1B1, causing clinically significant drug interactions. Thus, OATP1B1 plays a major role in the hepatic uptake of drugs, and genetic variants and drug interactions affecting OATP1B1 activity are important determinants of individual drug responses. In this article, we review the current knowledge about the expression, function, substrate characteristics, and pharmacogenetics of OATP1B1 as well as its role in drug interactions, in parts comparing with those of other hepatocyte-expressed organic anion transporting polypeptides, OATP1B3 and OATP2B1.